LSN2-T/30-D12 Series

DOSA-SIP, 30A POL DC/DC Converters

www.murata-ps.com

MDC_LSN2-T/30-D12 Series.B15 Page 6 of 17

Power-up Sequencing

If a controlled start-up of one or more LSN2-T/30-D12 Series DC/DC converters

is required, or if several output voltages need to be powered-up in a given

sequence, the On/Off control pin can be driven with an external open collector

device as per Figure 4.

Output Overvoltage Protection

LSN2-T/30-D12 SIP Series DC/DC converters do not incorporate output

overvoltage protection. In the extremely rare situation in which the device’s

feedback loop is broken, the output voltage may run to excessively high levels

continuously attempt to restart itself, go into overcurrent, and then shut down.

Once the output short is removed, the converter will automatically restart itself.

Output Reverse Conduction

Many DC/DCs using synchronous rectification suffer from Output Reverse

Conduction. If those devices have a voltage applied across their output before

a voltage is applied to their input (this typically occurs when another power

supply starts before them in a power-sequenced application), they will either

fail to start or self destruct. In both cases, the cause is the “freewheeling” or

“catch” FET biasing itself on and effectively becoming a short circuit.

LSN2-T/30-D12 SIP DC/DC converters do not suffer from Output Reverse

Conduction. They employ proprietary gate drive circuitry that makes them

immune to moderate applied output overvoltages.

Thermal Considerations and Thermal Protection

The typical output-current thermal-derating curves shown below enable

designers to determine how much current they can reliably derive from each

model of the LSN2-T/30-D12 SIPs under known ambient-temperature and air-

flow conditions. Similarly, the curves indicate how much air flow is required to

reliably deliver a specific output current at known temperatures.

The highest temperatures in LSN2-T/30-D12 SIPs occur at their output

were developed using thermocouples to monitor the inductor temperature and

varying the load to keep that temperature below +110°C under the assorted

conditions of air flow and air temperature. Once the temperature exceeds

+115°C (approx.), the thermal protection will disable the converter. Automatic

restart occurs after the temperature has dropped below +110°C.

As you may deduce from the derating curves and observe in the efficiency

curves on the following pages, LSN2-T/30-D12 SIPs maintain virtually constant

efficiency from half to full load, and consequently deliver very impressive

temperature performance even if operating at full load.

Lastly, when LSN2-T/30-D12 SIPs are installed in system boards, they are

obviously subject to numerous factors and tolerances not taken into account

here. If you are attempting to extract the most current out of these units under

demanding temperature conditions, we advise you to monitor the output-

inductor temperature to ensure it remains below +110°C at all times.

Start Up Considerations

When power is first applied to the DC/DC converter, operation is different than

when the converter is running and stabilized. There is some risk of start up

difficulties if you do not observe several application features. Lower output

voltage converters may have more problems here since they tend to have

higher output currents. Operation is most critical with any combination of the

any and all possible overvoltage situations, voltage limiting circuitry must be

provided external to the power converter.

Output Overcurrent Detection

Overloading the power converter’s output for an extended time will invariably

cause internal component temperatures to exceed their maximum ratings

and eventually lead to component failure. High-current-carrying components

such as inductors, FET’s and diodes are at the highest risk. LSN2-T/30-D12

SIP Series DC/DC converters incorporate an output overcurrent detection and

shutdown function that serves to protect both the power converter and its load.

If the output current exceeds it maximum rating by typically 50% or if the

output voltage drops to less than 98% of it original value, the LSN2-T/30-D12’s

internal overcurrent-detection circuitry immediately turns off the converter,

which then goes into a “hiccup” mode. While hiccupping, the converter will

+INPUT

+V

CONTROLLER

SMALL

SIGNAL

TRANSISTOR

HI = ON

LO = OFF

SHUTDOWN

SIGNAL

GROUND

COMMON

ON/OFF

CONTROL

Figure 4. On/Off Control Using An External Open Collector Driver